What to choose GPU or CPU , and should I use opencl, Linux (openSUSE)?
Long story short. Until Vulkan is implemented in Blender (which is going to be soon from what I hear) you can use OpenCL

Yes using OpenCL will reduce your render times significantly compared to when you have set it to 'None'.

But since 570x is not that fast compared to 2600X CPU, it comes down to very specific situations where you will have advantage in using GPU over CPU (while in other cases it would be opposite). It won't be as vast as in my case, as I have a pretty old CPU.

Using a rough estimate, you would get better results using CPU in cycles (an additional setting, not present in eevee), but make sure to set the tile size accordingly in Performance Submenu In Render Settings (right side on blender, not in preferences menu). A tile size of 128 or 256 or 512 is ideal for GPU (render something on each setting to test), and 16 or 32 is ideal for CPU.

Somethings only use CPU, like the cloth simulations.

Also Blender allows you to use CPU &amp; GPU together where possible. Just click the box in front of both CPU and GPU, like in my following screenshot.

Also, most of the blender devs use Linux, various flavors, so Linux is strongly supported. You can even get bleeding edge Alphas from builder.blender.org

Note that Blender would not raise or cause any errors based on what you are using CPU or GPU. You have to test it by doing different renders or better yet running the benchmark (BMW scene only .. or the comprehensive 5-7 scenes)

In my experience, for a beginner, you are better off just using the settings I showed below, and use eevee without worries.

Having installed ROCM recently myself on a seperate system (Ubuntu used for some unrelated testing).. I don't think you need AMD ROCM.But rather the drivers downloaded from AMD website (they released Linux drivers, that include OpenCL support), or alternatively try installing the xf86-video-amdgpu if you are using XORG